The fuel assemblies of nuclear reactors such as reactors cooled by pressurized water are built in the form of bundles of fuel rods of great length which are held in arrangements where the rods are parallel to each other, by means of a rigid framework closed by two end fittings.
Each of the fuel rods consists of a tubular cladding, generally made of a zirconium alloy containing pellets of sintered fuel material and closed at its ends by sealed plugs, so that the pellets of fuel materials are in contact with an inert gas under pressure, introduced into the cladding.
After a certain time of presence in the nuclear reactor in operation, some rods of the fuel assemblies of the core can become defective as a result of the formation of cracks in their cladding, under the effect of mechanical or chemical stresses to which these fuel rods are subjected.
Cracks passing through the cladding of the faulty fuel rods permit the escape of the gas contained in the cladding, which may contain radioactive products. The reactor cooling water is liable to come into direct contact with the pellets of fuel rods, and this can result in the damage or even in the complete destruction of these pellets and of the fuel rod.
The core fuel assemblies in which one or more fuel rods have leaks must therefore be detected very rapidly.
During the operations of reloading of the core of the nuclear reactor, the fuel assemblies are placed underwater in a pool, before being reloaded into the core or replaced with new assemblies.
Various procedures for monitoring fuel assemblies placed underwater in a pool have been proposed, so as to detect and/or to localize fuel rods exhibiting leaks in these assemblies.
The most commonly used solution consists in placing the assemblies extracted from a nuclear reactor into a cell where these assemblies are heated so that the pressure of the fission gases contained in the fuel rods increases and that these gases escape into the cell through the cracks in the faulty rods. However, this method does not make it possible to identify the defective rods which are to be replaced.
Furthermore, many techniques for detecting fuel pencils exhibiting leaks by means of ultrasonics have been proposed.
In particular, FR-A-2,538,155 discloses a process and a device for detecting defective fuel elements by an ultrasonic absorption method. A train of ultrasonic waves is transmitted in the cladding of the fuel element, from one end of this cladding consisting of a plug, at a frequency and for a duration chosen so that the propagation takes place as Lamb waves. The echoes are detected at the plug from which the ultrasonic waves are transmitted, in the case of different frequencies located in a frequency range, a fraction of which corresponds to a marked absorption by the water which may be present in the cladding and another to a considerable formation of an echo in a mechanical defect in the cladding.
In fact, in the case where a cladding is cracked, the pellets contained in this cladding are in contact with a film of water which seeped into the fuel rod when the pressure of the filling gas became sufficiently low. This film of water surrounding the pellets inside the cladding produces a very considerable attenuation of the Lamb waves in an S.sub.o mode, i.e., of the ultrasonic waves at a frequency such that they propagate in a plate mode within the cladding, in a longitudinal direction.
Insofar as these Lamb waves are only very slightly attenuated by water in which the rod cladding is immersed, or would be similarly attenuated very slightly by water filling the entire internal volume of the cladding, a high attenuation enables the presence of leaks in a fuel rod to be detected with certainty.
However, in the case where the transmission and the sensing of the ultrasonic waves are carried out in contact with the same sealing plug of the rod, generally the plug located at the upper end of the rod, the measured attenuation expresses, above all, the presence of water above the column of fuel pellets and below the plug, in a region where there is a spring retaining the fuel pellets. The detection is therefore not highly sensitive in the case of small-sized cracks, and the detection method must be supplemented by ultrasonic detection of conventional type, using echoes from the mechanical defects in the cladding.
It is possible to employ an ultrasonic transducer placed directly in contact with the upper end of the plugs of each of the rods of the assembly, after disassembly of the upper end fitting, or else ultrasonic transducers fixed to blade-shaped devices called sabers, which can be slid between the rows of fuel rods. In the latter case, there is no need to disassemble the upper end fitting of the assembly.
However, as indicated above, there is a risk that the detection method may lack sensitivity or be tricky to apply and use.